1. Field of the Invention
This invention relates to removal of cores from castings and more particularly to removal of sacrificial cores by electrochemical machining.
2. Brief Description of the Prior Art
In the manufacture of complex parts from refractory metals, it is sometimes necessary to provide recesses and/or passages within the body of the casting. Forming such recesses or passages by conventional mechanical machining may be difficult, impractical or even impossible for some refractory materials and/or some geometrical shapes of the casting. Accordingly, processes have been developed to produce such internal structures by casting. Typically, a sacrificial core is positioned within the mold in which the article is cast. The mold is then filled with molten metal and cooled to solidify the metal. The mold is then opened and the part is removed. However, ordinarily the core must be separately removed. In many cases the shape of the core prevents it from being removed in one piece, especially in the case of precision castings, where the final dimensions of the article are formed in the mold. Consequently, the use of sacrificial cores that are removed by destroying them has been developed.
Ceramic cores have been used in casting refractory metal parts. However, the removal of the hard ceramic must be accomplished by mechanical means, which are time-consuming and costly. Accordingly, it has been proposed to use metal cores that can be removed by chemical leaching. For example, the use of steel cores has been proposed for casting articles from refractory alloys of cobalt, nickel, titanium and the like. The steel cores may be removed by chemical leaching with strong acids such as nitric and hydrochloric acids. However, even with the use of such corrosive materials, removal of the cores by chemical leaching is extremely slow and difficult, especially in deep recesses and narrow passages.
Accordingly, a need has continued to exist for a method of removing sacrificial cores from castings that is simpler and more rapid than methods hitherto employed.